92 HUMAN PHYSIOLOGY 



waste matters which the gland passes upwards to the 

 skin surface. Sweat glands are most numerous in 

 the palm of the hand and sole of the foot, the num- 

 ber in these regions being estimated at about 3,000 

 per square inch. In the neck and back they are less 

 numerous. It has been estimated that over two 

 millions of these glands exist in the skin surface of a 

 human body. If the coiled up tube was stretched 

 out at full length it would measure about a quarter 

 of an inch, so that according to one estimate, in a 

 square inch of skin from the palm of the hand the 

 length of the sweat tubing would be found to be over 

 seventy-three feet. The total length of sweat tubing 

 in the body has been variously given as amounting 

 to ten miles, some estimates vastly exceeding this 

 calculation. 



THE SKIN ACTION. The skin glands are always 

 acting, and hence perspiration which is poured forth 

 under circumstances of quietude of the body is 

 termed insensible perspiration. After exertion, when 

 the sweat may become visible on the surface of the 

 skin, it is then known as sensible perspiration. The 

 amount of sweat given off on an average from the 

 body of a man per day is about two pounds, an 

 amount liable to be vastly increased where violent 

 exercise or heavy work is represented. 



THE SKIN AND BODILY HEAT. In order to appre- 

 ciate the duty of the skin as a regulator of the 

 temperature or heat of the body, we have first of all 

 to remember the vast amount of blood which is 

 perpetually circulating through the skin in its minute 

 bloodvessels or capillaries. These bloodvessels are 

 under the control of the nervous system, and are 

 maintained in a medium state which may be described 

 as that between contraction and expansion. If, 



